A star schema is a type of database organization where one or more fact tables reference a number of dimension tables. A fact table typically records measurements or metrics for an event, and includes values and foreign keys to dimension tables where descriptive information is stored. The star schema gets its name from the physical model's resemblance to a star with a fact table at its center and the dimension tables surrounding the fact table representing the star's points. The star schema is a specialized form of a snowflake schema, which also has fact tables, but in the snowflake schema, dimensions are normalized into multiple related tables, whereas the star schema's dimensions are denormalized with each dimension represented by a single table.
Fact tables often include a low level of granularity, which often results in fact tables growing quite large over time. When performing a database query on a database using a star or snowflake schema, a join operation is typically performed on a fact table and one or more of the dimension tables. Due to the size of the fact tables, however, such join operations can be very time and processor-intensive.